1. Field of the Invention
This invention relates to improvements in an interchangeable lens for a camera capable of selectively performing automatic focusing and motor-driven manual focusing.
2. Description of the Related Art
In the case of an interchangeable lens for a camera having drive means (such as a motor) for driving the focusing lens to thereby make it possible to selectively performing automatic focusing and motor-driven manual focusing (hereinafter referred to as "power" focusing), it has been the general rule that the power focusing is prohibited/allowed by the auto/manual changeover switch in such an arrangement as shown in FIG. 6.
In FIG. 6, 1 is a camera body; 2 is a camera side control circuit; 3 is a quick return half-mirror; 4 is a submirror; 5 is an AF sensor 5 comprised of a CCD, etc.; 6 is a signal line for the AF sensor drive signal; 7 is a signal line for the AF sensor output; 8 is a film plane; 9 is a pentagonal roof type prism. 10 is an interchangeable lens; 11 is a lens control circuit; 12 is a focusing lens; 13 is an electric motor which drives the focusing lens 12; 14 is a pulse disc which rotates as the focusing lens 12 moves; 15 is a constant current source; 16 is a light-emitting diode for projecting light to the pulse disc 14 to detect the amount of movement of the focusing lens 12; 17 is a phototransistor which receives the light reflected from the pulse disc 14; 18 is an auto/manual changeover switch which, when turned on, selects the power focusing (manual focusing) and when off selects the automatic focusing; 19 is a power focusing switch which when a forward-moving side contact point 19a turns on, selects forward movement of the focusing lens 12, and when a backward-moving side contact point 19b turns on, selects backward movement of the focusing lens 12; 20-22 are pull-up resistors; 23 is a communication line connecting the canera side control circuit 2 to the lens control circuit 11.
In such an arrangement as above, the lens control circuit 11 uses, as a forward-moving signal and a backward-moving signal, signals of low level due to the ON state of the forward-moving side contact 19a and the backward-moving side contact 19b of the power focusing switch 19. When either one of the inputs to a forward-moving terminal and a backward-moving terminal becomes low level, the lens control circuit 11 supplies electric power to the motor 13 to drive the focusing lens 12 in the corresponding direction. The forward-moving side contact 19a and the backward-moving side contact 19b of the power focusing switch 19 are normally made high level by the pull-up resistors 20 and 21. But, as the power focusing operation is carried out, when either one of the forward-moving side contact 19a and the backward-moving side contact 19b turns on, in other words, is connected to a movable contact 19c, the potential level at the contact connected to the movable contact 19c depends on the state of the auto/manual changeover switch 18. That is, when the auto/manual changeover switch 18 is off (automatic focusing), it becomes high level, and when on (manual focusing), it becomes low level.
Thereby, when the auto/manual changeover switch 18 is changed over to automatic focusing, the input signals to the forward-moving terminal and the backward-moving terminal of the lens control circuit 11 both maintain the high level state, not depending on the operated state of the power focusing switch 19. When the auto/manual changeover switch 18 is changed over to the manual focusing, the forward-moving side contact 19a or the backward-moving side contact 19b which is turned on by the operation of the power focusing switch 19 is connected to the ground through the movable contact 19c and the auto/manual changeover switch 18, becoming low level. Therefore, a forward-moving or backward-moving signal of low level enters the lens control circuit 11. Responsive to this, the motor 13 carries out forward or backward axial movement of the focusing lens 12.
In such a manner, the prohibition/allowance of the power focusing is changed over by the ON/OFF state of the auto/manual changeover switch 18 is provided in the interchangeable lens 10 without variation. Therefore, power focusing is determined irrespective of the camera body 1.
In the conventional example shown in FIG. 6, the auto/manual changeover corresponds to whether or not the power focusing is usable in one-to-one relation. This, if the ordinary camera is considered, seems to fulfill the necessity and sufficiency. Yet, when the automatic focusing is unable to detect the in-focus state, or its accuracy of focus detection is low, that is, in the case of the passive type automatic focusing, when an object to be photographed is in low illumination or of low contrast, or in the case of the active type automatic focusing when the reflectance of the object is very low, or the object distance is very long, it is desirable for the user to carry out focusing manually. For this situation, it is desirable that without having again to manipulate the auto/manual changeover switch, changing over to power focusing takes place automatically. Also, the person of high experience has a demand that after the in-focus state is obtained by automatic focusing, a sharper focus adjustment can be obtained manually. Even for this case, it is desirable that without manipulating the auto/manual changeover switch, automatic switching to power focusing occurs.
But, though this capability is very advantageous to the professional who thoroughly knows the camera, there is produced a possibility for the beginner who is not well accustomed with management of the camera to get all the worse for the faulty operation of the power focusing switch to spoil the effort to establish the in-focus.
Such problems are attributable to the fact that the selection of the prohibition and allowance of the power focusing relies only on the ON/OFF of the auto/manual changeover switch and is not controlled from the camera body.